fix flake8 issues
fix flake8 issues

pydata/xarray,markelg/xray,shoyer/xarray,markelg/xray,pydata/xarray,jcmgray/xarray,jhamman/xarray,jhamman/xarray,rabernat/xray,jhamman/xarray,pydata/xarray,rabernat/xray,xray/xray,chunweiyuan/xarray,jhamman/xray,shoyer/xarray,markelg/xray,shoyer/xray